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"First up I said 'Nah, it couldn't be'. Then I saw his photo. What he's done, it's not right," Brenton Tarrant's grandmother Marie Fitzgerald told Nine News.
Tarrant has been arrested and charged by New Zealand Police with murder following the terrorist attack in Christchurch on Friday afternoon.
Speaking to Australian media on Sunday, his 81-year-old grandmother says his entire family is devastated he could do something like this.
"We're all gobsmacked, we don't know what to think. The media saying he's planned it for a long time so he's obviously not of sound mind I think," she said.
"We're so sorry, for the families over there, for the dead and the injured...[we] just want to go home and hide.
"It's just so much of everything to take in that somebody in our family would do anything like this."
Fifty people have died following the attack on two mosques in Christchurch city during the worshippers' prayer time.
Following Tarrant's arrest, his mother and sister are reportedly staying in a safe house in Australia and assisting authorities, Nine News reports.
In his teenage years, Fitzgerald said her grandson spent his free time in his room playing video games.
"He spent most of his time on computers... and playing computer games," she said.
"It's only since he travelled overseas I think that that boy has changed completely [from] the boy we knew. Now everybody's just devastated."
Elsewhere, more than $6.3 million has now been donated to the victims of the Christchurch massacre via online fundraising pages.